Crystallization

A crystal is a solid whose particles are arranged in a regular, repeating pattern. You know many: rock salt, mishri (candy sugar), snowflakes, frost on a window. They also form in caves, such as Mawsmai Cave in Sohra (Meghalaya), and as quartz in rocks.

How crystals form, with numbers. Make a saturated solution of compound B at 60 °C: 287 g dissolved in 100 g of water. Now cool it to 40 °C, where only 241 g can stay dissolved. The extra, 287 – 241 = 46 g, comes out as pure crystals.

This is crystallization: getting crystals from a saturated solution. It is used to separate a solid present in small amounts and to purify solids. It works because solubility changes with temperature. Note that it is the solute that forms crystals; the water stays liquid. You have seen this at the sweet shop, where boiled-down sugar syrup (chashni) turns into crystals.

Making copper sulfate crystals:

• Dissolve copper sulfate in water with a drop of dilute sulfuric acid (your teacher adds the acid), warming it over a water bath until no more dissolves.
• Filter the hot solution.
• Cover it and let it cool slowly.
• Filter out the crystals, rinse them with cold water and dry them.

Copper sulfate is poisonous: do not touch it.

"How do you get big crystals?" Cool (or evaporate) slowly: you get larger, well-shaped crystals. Cool quickly and you get small, less neat ones.

Salt from the sea. In shallow salt pans by the sea, the Sun evaporates the water until it is saturated and salt crystallises out. Along India's coasts, people once made panga salt by boiling down concentrated salt water, and karkatch salt by letting seawater evaporate.